Non-Oriented Electrical Steel Sheet Having Superior Magnetic Properties and a Production Method Therefor

ABSTRACT

A non-oriented electrical steel sheet having outstanding magnetic properties and i, as percentages by weight, from 0.7˜3.02.5% of Al, 0.2˜32.5% of Si, 0.2˜2.01.5% of Mn, 0.001 0005˜0.004% of N, 0.0005˜0.004% of S, 0.003% or less of C, and a balance of Fe and other unavoidably incorporated impurities, wherein the Al, Mn, N and S are included so as to satisfy the compositional formulae 1≤{[Al]+[Mn]}≤3.5, 0.002≤{[N]+[S]}≤0.006, 300250≤{([Al]+[Mn])/([N]+[S])}≤1,400, 1.4≤{[Al]+[Si]+[Mn]/2}≤5.5, 1 [Al]/[Si]≤7.5, 1≤[Al]/[Mn]≤3.7; and a production method therefor. By optimising the Al, Si, Mn, N and S added components, the distribution density of coarse inclusions is increased, thereby improving the crystal-grain growth properties and domain wall motility and producing the highest grade of non-oriented electrical steel sheet having superior magnetic properties, low hardness, and superior customer workability and productivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/514,312 filed on Jun. 7, 2012, which is the United States nationalphase of International Application No. PCT/KR2010/009380 filed Dec. 28,2010, which claims priority to Korean Patent Application No.10-2009-0131990 filed Dec. 28, 2009, Korean Patent Application No.10-2009-0131992 filed Dec. 28, 2009, Korean Patent Application No.10-2010-0135003 filed Dec. 24, 2010, Korean Patent Application No.10-2010-0135004 filed Dec. 24, 2010, and Korean Patent Application No.10-2010-0135943 filed Dec. 27, 2010, the disclosures of which are herebyincorporated in their entirety by reference.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to the production of a non-orientedelectrical steel sheet, and particularly to a non-oriented electricalsteel sheet of the highest quality, wherein the components of steel areoptimally designed to increase the distribution density of coarseinclusions in steel and to improve growth of grains and mobility ofdomain walls, so that magnetic properties are enhanced, and low hardnessis ensured, thus improving productivity and punchability, and to amethod of producing the same.

Background Art

The present invention pertains to the production of a non-orientedelectrical steel sheet useful as a material for iron cores of rotationdevices. This non-oriented electrical steel sheet is essential in termsof converting electrical energy into mechanical energy, and thus themagnetic properties thereof are regarded as very important. The magneticproperties mainly include core loss and magnetic flux density. Becausethe core loss is energy that disappears in the form of heat in thecourse of converting energy, it is good for it to be as low as possible.The magnetic flux density is a power source of a rotator. The higher themagnetic flux density, the more favorable the energy efficiency.

Typically, a non-oriented electrical steel sheet is composed mainly ofSi in order to reduce core loss. When the amount of Si increases, themagnetic flux density decreases. If the amount of Si is excessivelyincreased, processability is decreased making it difficult to performcold rolling. Furthermore, the lifetime of a mold may decrease uponpunching by the customer. Hence, attempts are made to decrease theamount of Si and increase the amount of Al so as to improve magneticproperties and mechanical properties. However, the magnetic propertiesof non-oriented electrical steel sheet of the highest quality are notobtained, and such sheets have not yet been actually produced because ofdifficulties in mass producing them.

Meanwhile, to obtain a non-oriented electrical steel sheet with goodmagnetic properties, impurities including C, S, N, Ti and so on such asfine inclusions present in steel are controlled to be minimal and thusthe growth of grains needs to be increased. However, the control ofimpurities to the minimum is not easy in a typical production process ofelectrical steel sheets, and the cost of a steel making process mayundesirably increase.

The impurities which were not removed in the steel making process arepresent in the form of nitrides or sulfides in a slab upon continuouscasting. As the slab is re-heated to 1,100° C. or higher for hotrolling, inclusions such as nitrides or sulfides may be re-dissolved andthen finely precipitated again upon termination of hot rolling.

The inclusions that are precipitated in typical non-oriented electricalsteel sheets include MnS and AlN, which are observed to have a smallaverage size of about 50 nm, and such fine inclusions may hinder thegrowth of grains upon annealing thus increasing hysteresis loss andobstructing the movement of domain walls upon magnetization, undesirablylowering permeability.

Therefore, in the process of producing the non-oriented electrical steelsheet, impurities are appropriately controlled from the steel makingprocess so that such fine inclusions are not present, and the residualinclusions should be prevented from being more finely precipitated viare-dissolution upon hot rolling.

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the related art, and an object of thepresent invention is to provide a non-oriented electrical steel sheet ofthe highest quality, wherein the proportions of Al, Si and Mn which arealloy elements of steel and N and S which are impurity elements of steelare optimally controlled so that the distribution density of coarseinclusions in steel is increased and the formation of fine inclusions isdecreased, thus enhancing the growth of grains and the mobility ofdomain walls to thereby manifest excellent magnetic properties, and alsosuperior productivity and punchability because of low hardness.

SUMMARY OF THE INVENTION

In order to accomplish the above object, an aspect of the presentinvention provides a non-oriented electrical steel sheet having superiormagnetic properties, comprising 0.7˜2.5% of Al, 0.2˜2.5% of Si, 0.2˜1.5%of Mn, 0.0005˜0.004% of N, 0.0005˜0.004% of S, 0.003% or less of C, anda balance of Fe and other inevitable impurities by wt %, and satisfyingthe conditions below:

Conditions: 1.0≤[Al]≤3.0, 0.5≤[Si]≤2.5, 0.5≤[Mn]2.0, 1≤{[Al]+[Mn]}≤3.5,0.002≤{[N]+[S]}≤0.006, 300250≤{([Al]+[Mn])/([N]+[S])}≤1,400,1.4≤{[Al]+[Si]+[Mn]/2}≤5.5, 1≤[Al]/[Si]≤7.5, 1≤[Al]/[Mn]≤3.7, wherein[Al], [Si], [Mn], [N] and [S] indicate amounts (wt %) of Al, Si, Mn, Nand S, respectively.

The non-oriented electrical steel sheet may further comprise 0.2% orless of P.

The non-oriented electrical steel sheet may further comprise at leastone of 0.005˜0.2% of Sn and 0.005˜0.1% of Sb.

The non-oriented electrical steel sheet may further comprise 0.004% orless of Ti.

The non-oriented electrical steel sheet which satisfies Condition mayhave inclusions comprising nitrides and sulfides alone or combinationsthereof formed in the steel sheet.

The nitride may comprise aluminum nitride and the sulfide may comprisemagnesium sulfide.

The inclusion may have an average size of 300 nm or more.

A distribution density of the inclusion may having an size of 300 nm ormore may be equal to or greater than 0.02 number/mm²

A cross-sectional Vickers hardness (Hv1) may be 190 or less.

According to the present invention, the proportions of alloy elementssuch as Al, Si and Mn and of impurity elements such as N and S can beappropriately controlled so as to increase the distribution density ofcoarse inclusions, thus enhancing the growth of grains and the mobilityof domain walls. Thereby, a non-oriented electrical steel sheet of thehighest quality having excellent magnetic properties and very lowhardness can be stably produced. Also customer workability andproductivity are superior, and the unit cost of production of productscan be decreased, thus reducing the cost.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view showing composite inclusions which are present in anon-oriented electrical steel sheet according to the present invention.

DESCRIPTION OF THE INVENTION

To solve the technical problems as mentioned above, the presentinventors have examined the effects of alloy elements and impurityelements in steel and of the relation between respective elements onforming the inclusions and also the effects thereof on magneticproperties and processability, resulted in the finding that among alloyelements of steel, the amounts of Al, Si and Mn and the amounts ofimpurity elements such as N and S may be appropriately adjusted andAl/Si and Al/Mn, Al+Si+Mn/2, Al+Mn, N+S and (Al+Mn)/(N+S) may beoptimally controlled so that the hardness of a steel sheet is decreasedand the distribution density of coarse composite inclusions having anaverage size of 300 nm or more in the steel sheet is increased, therebydrastically enhancing magnetic properties and improving productivity andpunchability, which culminates in the present invention.

The present invention is directed to a non-oriented electrical steelsheet of the highest quality, comprising 0.7˜2.5% of Al, 0.2˜2.5% of Si,0.2˜1.5% of Mn, 0.0005˜0.004% of N, 0.0005˜0.004% of S, 0.003% or lessof C, and a balance of Fe and other inevitable impurities by wt %,wherein Al, Si, Mn, N and S are contained so as to satisfy the followingCondition and thus the distribution density of 300 nm or more sizedcoarse inclusions having combinations of nitrides and sulfides isincreased to be equal to or greater than 0.02 number/mm², resulting inhigh magnetic properties and low hardness.

Condition: 1≤{[Al]+[Mn]}≤3.5, 0.002≤{[N]+[S]}≤0.006,250≤{([Al]+[Mn])/([N]+[S])}≤1,400, 1.4≤{[Al]+[Si]+[Mn]/2}≤5.5,1≤[Al]/[Si]≤7.5, 1≤[Al]/[Mn]≤3.7

As such, [Al], [Si], [Mn], [N] and [S] indicate the amounts (wt %) ofAl, Si, Mn, N and S, respectively.

In addition, the present invention is directed to the production of thenon-oriented electrical steel sheet which is superior in both magneticproperties and processability, by adding 0.3˜0.5% of Al to molten steelto perform deoxidation in a steel making process, adding remaining alloyelements, and then maintaining the temperature of the molten steel at1,500˜1,600° C. thus manufacturing a slab having the composition thatsatisfies Condition, followed by heating the slab to 1,100˜1,250° C. andthen performing hot rolling wherein finish hot rolling is conducted at800° C. or higher, carrying out cold rolling, and then finally annealingthe cold rolled sheet at 750˜1,100° C.

The alloy elements of steel, namely, Al, Si and Mn are described below.These alloy elements are added to reduce the core loss of an electricalsteel sheet. As the amounts thereof increase, the magnetic flux densitymay decrease and the processability of a material may deteriorate.Hence, the amounts of such alloy components are appropriately designedto improve not only the core loss but also the magnetic flux density,and also hardness needs to be maintained to an appropriate level orless.

Furthermore, Al and Mn combine with N and S which are impurity elementsto form inclusions such as nitrides or sulfides. Such inclusions greatlyaffect magnetic properties and thus the formation of inclusions thatminimize the deterioration of magnetic properties should be increased.

The present inventors were the first to discover that coarse compositeinclusions comprising combinations of nitrides or sulfides may be formedwhen the amounts of Al, Mn, Si, N and S are adapted for specificconditions, and have found the fact that the distribution density ofsuch composite inclusions to a predetermined level or more is ensured,and thereby magnetic properties may be drastically improved despite theaddition of minimum amounts of alloy elements that deteriorateprocessability, and thus devised the present invention.

The reason why the ranges of component elements of the present inventionand the amount ratios of the component elements are limited is describedbelow.

[Al: 0.7˜2.5 wt %]

Al functions to increase resistivity of a material to reduce core lossand to form a nitride, and is added in an amount of 0.7˜2.5% so as toform a coarse nitride. If the amount of Al is less than 0.7%, inclusionsmay not be sufficiently grown. In contrast, if the amount thereofexceeds 2.5%, processability may deteriorate and all processes includingsteel making, continuous casting and so on may be problematic, making itimpossible to produce a steel sheet in the typical manner.

[Si: 0.2˜2.5 wt %]

Si functions to increase resistivity of a material to reduce core loss.If the amount of Si is less than 0.2%, it is difficult to expectreduction effects of core loss. In contrast, if the amount thereofexceeds 2.5%, the hardness of a material may increase, undesirablydeteriorating productivity and punchability.

[Mn: 0.2˜1.5 wt %]

Mn functions to increase resistivity of a material to reduce core lossand to form a sulfide, and is added in an amount of 0.2% or more. If theamount thereof exceeds 1.5%, the formation of [111] texture that isunfavorable for magnetic properties may be facilitated. Hence, theamount of Mn is preferably limited to 0.5˜1.5%.

[Sn: 0.005˜0.2 wt %]

Sn is preferentially segregated on the surface and the grain boundariesand may reduce accumulated strain energy upon hot rolling and coldrolling, so that the strength in {100} orientation that is favorable formagnetic properties may increase whereas the strength in {111}orientation that is unfavorable for magnetic properties may decrease,thus achieving improvements in texture. Hence, Sn is added in the rangeof 0.2% or less. Furthermore, Sn is preferentially formed on the surfaceduring welding to thus suppress surface oxidation and enhance the weldproperties thereby increasing productivity of continuous lines. Also,the formation of Al-based oxides and nitrides on the surface or thelayer under the surface may be suppressed during heat treatment, thusenhancing magnetic properties. Upon punching by a customer, the increasein hardness of the layer under the surface due to nitrides may beinhibited to improve punchability.

Hence, Sn is preferably added in the range of 0.005% or more. Incontrast, if the amount of Sn exceeds 0.2%, improvements in magneticproperties based on such an additional use thereof are insignificant,and fine inclusions and deposits may be formed in steel, rather thanpreferential segregation on the surface and the grain boundaries,negatively affecting the magnetic properties. Also, cold rollability andpunchability may decrease and the Erichsen number that represents theweld properties is 5 mm or less, making it impossible to perform weldingof the same species. Thus, a low-graded material having the sum of Siand Al of less than 2 should be undesirably used as a connectionmaterial for continuous line working. Hence, the amount of Sn ispreferably limited to 0.005˜0.2%.

[Sb: 0.005˜0.1 wt %]

Sb is preferentially segregated on the surface and the grain boundariesand may reduce accumulated strain energy upon hot rolling and coldrolling, so that the strength in {100} orientation that is favorable formagnetic properties may increase and the strength in {111} orientationthat is unfavorable for magnetic properties may decrease, thus attainingimprovements in texture. Hence, Sb is added in the range of 0.1% orless. Furthermore, Sb is preferentially formed on the surface duringwelding to thus suppress surface oxidation and enhance weld propertiesthereby increasing productivity of continuous lines. Also, the formationof Al-based oxides and nitrides on the surface or the layer under thesurface may be suppressed during heat treatment, thus improving magneticproperties. Upon punching by the customer, the increase in hardness ofthe layer under the surface due to nitrides may be inhibited to improvepunchability.

Hence, Sb is preferably added in the range of 0.005% or more. Incontrast, if the amount of Sb exceeds 0.1%, improvements in magneticproperties based on such an additional use thereof are insignificant,and fine inclusions and deposits may be formed in steel, rather thanpreferential segregation on the surface and the grain boundaries,undesirably aggravating the magnetic properties. Also, cold rollabilityand punchability may decrease and the Erichsen number that representsthe weld properties is 5 mm or less, making it impossible to performwelding of the same species. Thus, a low-graded material having the sumof Si and Al of less than 2 should be undesirably used as a connectionmaterial for continuous line working. Hence, the amount of Sb ispreferably limited to 0.005˜0.1%.

[P: 0.2 wt % or less]

When P is added in the range of 0.2% or less, texture that is favorablefor magnetic properties may be formed, and in-plane anisotropy andprocessability are improved. If the amount thereof exceeds 0.2%, coldrollability may decrease and processability may deteriorate. Hence, theamount of P is limited to 0.2% or less.

[N: 0.0005˜0.004 wt %]

N is an impurity element, and may form a fine nitride during theproduction process to suppress the growth of grains undesirablydeteriorating core loss. Although the formation of nitrides issuppressed, an additional high cost and long process time are required,and thus monetary benefits are negatively affected. Therefore, it ispreferred that an element having high affinity for the impurity elementN is positively utilized to coarsely grow inclusions so as to reduce aninfluence on the growth of grains. To coarsely grow the inclusions inthis way, the amount of N is essentially controlled in the range of0.0005˜0.004%. If the amount of N exceeds 0.004%, the inclusions may notbe coarsely formed undesirably increasing core loss. More preferably,the amount of N is limited to 0.003% or less.

[S: 0.0005˜0.004 wt %]

S is an impurity element, and may form a fine sulfide during theproduction process to thus suppress the growth of grains and deterioratecore loss. Although the formation of sulfides is suppressed, anadditional high cost and long process time are required, and thusmonetary benefits are negatively affected. Thus, it is preferred that anelement having high affinity for the impurity element S is positivelyutilized to coarsely grow inclusions so as to reduce the influence onthe growth of grains. To coarsely grow the inclusions in this way, theamount of S is essentially controlled in the range of 0.0005˜0.004%. Ifthe amount of S exceeds 0.004%, the inclusions may not be coarselyformed undesirably increasing core loss. More preferably, the amount ofS is limited to 0.003% or less.

In addition to the above impurity elements, inevitable impurities suchas C, Ti may be incorporated. C may cause magnetic aging, and the amountthereof is thus limited in the range of 0.003% or less. Ti may promotethe growth of [111] texture that is unfavorable for a non-orientedelectrical steel sheet, and the amount thereof is thus limited in therange of 0.004% or less, and preferably 0.002% or less.

In the non-oriented electrical steel sheet that satisfies Condition, thesum ([Al]+[Mn]) of Al and Mn by wt % is limited to 3.5% or less. If thesum of Al and Mn exceeds 3.5% in steel comprising 0.7˜2.5% of Al,0.2˜2.5% of Si and 0.2˜1.5% of Mn, the fraction of [111] texture that isunfavorable for magnetic properties may increase undesirablydeteriorating the magnetic properties. In the non-oriented electricalsteel sheet that satisfies Condition, if the sum of Al and Mn is lessthan 1%, nitrides, sulfides or composite inclusions of these two are notcoarsely formed, thus deteriorating the magnetic properties.

In the present invention, the sum ([N]+[S]) of N and S is limited to0.002˜0.006%. This is because inclusions are coarsely formed in theabove range. If the sum of N and S exceeds 0.006%, the fraction of fineinclusions may be increased, undesirably deteriorating the magneticproperties.

Also in the present invention, the ratio of the sum ([Al]+[Mn]) of Aland Mn by wt % to the sum ([N]+[S]) of N and S by wt % is regarded asimportant.

The present inventors have appreciated that, in order for thedistribution density of 300 nm or more sized coarse composite inclusionsof nitrides and sulfides to increase and become equal to or greater than0.02 number/mm², ([Al]+[Mn])/([N]+[S]) should be appropriately adjusted,and the proper range of ([Al]+[Mn])/([N]+[S]) may vary depending on theamounts of Si, Al and Mn.

In the case where the amounts of Al, Si and Mn are given as inCondition, when the ratio of ([Al]+[Mn])/([N]+[S]) is 250˜1400, theformation of composite inclusions may be effectively increased.Specifically, when the ratio of ([Al]+[Mn])/([N]+[S]) falls in the rangeof 250˜1400 under Condition, inclusions may be coarsely formed thusincreasing the distribution density of coarse composite inclusions. Incontrast, when the ratio thereof falls outside the above range, theinclusions are not coarsely formed and the formation of coarse compositeinclusions is low and texture that is unfavorable for magneticproperties is formed.

FIG. 1 shows composite inclusions which are present in the non-orientedelectrical steel sheet according to the present invention. When theamounts of Al, Mn, N and S are controlled in the optimal ranges,inclusions are grown several times or more compared to when usingtypical materials, thus increasing the formation of coarse compositeinclusions having an average size of 300 nm or more. Accordingly, theformation of fine inclusions having an average size of about 50 nm maydecrease, thereby improving magnetic properties. The present inventorshave appreciated that, when the distribution density of coarse compositeinclusions as shown in FIG. 1 is equal to or greater than 0.02number/mm², the magnetic properties of the non-oriented electrical steelsheet may be remarkably improved.

The accurate mechanism for forming such coarse composite inclusions hasnot yet been revealed, but is assumed to take place in the steel makingprocess. Specifically upon initial addition of Al in the steel makingprocess, Al-based oxides and nitrides may be formed due to deoxidation,and in the composition that additionally includes the alloy elementssuch as Al and Mn and satisfies the amounts of Al, Mn, Si, N and S asdesigned in the present invention upon bubbling, Al-based oxides andnitrides are grown and Mn-based sulfides may also be precipitatedthereon.

In the range of the present invention that satisfies Condition, namely,wherein the sum ([Al]+[Mn]) of Al and Mn by wt % is 3.5% or less and thesum ([N]+[S]) of N and S by wt % is 0.002˜0.006 and the ratio of the sumof Al and Mn to the sum of N and S ([Al]+[Mn])/([N]+[S]) falls in therange of 250˜1,400, inclusions are coarsely formed and the distributiondensity of coarse composite inclusions having an average size of 300 nmor more is greater than 0.02 number/mm², thus exhibiting superiormagnetic properties. However, in the range falling outside the presentinvention (outside the thick line), coarse inclusions are not formed andthe distribution density of coarse composite inclusions having anaverage size of 300 nm or more is less than 0.02 number/mm², thusdeteriorating texture and magnetic properties.

Although the coarse inclusions are mainly observed to be combinations ofnitrides and sulfides having an average size of 300 nm or more, theexamples thereof may include combinations of a plurality of nitrides orcombinations of a plurality of sulfides having an average size of 300 nmor more, and those having nitrides or sulfides alone having a size of300 nm or more. Herein, the average size of the inclusions is determinedby measuring the longest length and the shortest length of theinclusions when viewed in the cross-section of the steel sheet andaveraging the measured values.

Also in the non-oriented electrical steel sheet that satisfiesCondition, the amount ratio of Al to Si ([Al]/[Si]) is limited to 1˜7.5.In the case where the amount ratio of Al to Si is 1˜7.5, the grains mayeffectively grow and the hardness of a material may decrease thusimproving productivity and punchability. If the ratio of [Al]/[Si] isless than 1, inclusions do not greatly grow undesirably decreasing thegrowth of grains and deteriorating magnetic properties. Furthermore, theamount of Si may increase, undesirably enhancing hardness. If the ratioof [Al]/[Si] exceeds 7.5, texture of a material may become poorundesirably deteriorating the magnetic flux density.

In the present invention, the ratio of Al to Mn ([Al]/[Mn]) ispreferably limited to 1˜3.7. When the ratio of Al to Mn is 1˜3.7, theinclusions may effectively grow thus exhibiting superior core lossproperties. In contrast, if the ratio thereof falls outside the aboverange, the growth of inclusions may decrease and the fraction of texturethat is favorable for magnetic properties may decrease.

The limited ratio of alloy components related to resistivity isdescribed below. Recently, as the demand for environmentally friendlyautomobiles drastically increases, there is a high need for non-orientedelectrical steel sheets usable for highly rotatable motors. The motorsused in the environmentally friendly automobiles should greatly increasetheir number of rotations. When the number of rotations of the motor isincreased, the fraction of eddy current loss in the inner core loss maybe drastically increased. To reduce such eddy current loss, resistivityshould increase.

The relation between the amounts of alloy elements of the non-orientedelectrical steel sheet and the intrinsic resistance is representedbelow.

ρ=13.25+11.3([Al]+[Si]+[Mn]/2)(ρ:resistivity,Ω·m)

In the present invention that satisfies Condition, [Al]+[Si]+[Mn]/2 islimited to 1.7 or more so as to ensure the resistivity of 32 or more.Furthermore, in the present invention that satisfies Condition,[Al]+[Si]+[Mn]/2 is controlled to 5.5% or less so that resistivity(intrinsic resistance) is maintained to 75 or less and Vickers hardness(Hv1) is 190 or less.

Also the demand for high magnetic flux density products is drasticallyincreasing these days to achieve high efficiency of motors. Accordingly,there is an urgent requirement for non-oriented electrical steel sheetshaving lowered resistivity and improved magnetic flux density. In thecase where magnetic flux density properties are regarded as important,resistivity (intrinsic resistance) is decreased to 36 or less toincrease the magnetic flux density. Moreover to correspond to high-speedrotations, the resistivity should be controlled to at least 25.

Below is a description of a method of producing the non-orientedelectrical steel sheet according to the present invention. The method ofproducing the non-oriented electrical steel sheet preferably includesadding 0.3˜0.5% of Al, corresponding to a portion of the total amount ofadded Al, in the steel making process, so that deoxidation of steelsufficiently occurs, and adding the remaining alloy elements.Subsequently, the temperature of molten steel is maintained at1,500˜1,600° C. so that inclusions in steel are sufficiently grown,after which the resultant steel is solidified in a continuous castingprocess thus manufacturing a slab.

Subsequently, the slab is placed in a furnace so that it is re-heated to1,100˜1,250° C. If the slab is heated to a temperature exceeding 1,250°C., deposits that negatively affect the magnetic properties may bere-dissolved, hot rolled and then finely deposited, and thus the slab isheated to 1,250° C. or less.

Subsequently, the heated slab is hot rolled. Upon hot rolling, finishhot rolling is preferably carried out at 800° C. or more. The hot rolledsheet is annealed at 850˜1,100° C. If the annealing temperature of thehot rolled sheet is lower than 850° C., texture does not grow or finallygrows, and thus the extent of increasing the magnetic flux density islow. In contrast, if the annealing temperature of the hot rolled sheetis higher than 1,100° C., magnetic properties may deteriorate instead,and rolling workability may decrease due to plate transformation. Hence,the temperature range thereof is limited to 850˜1,100° C. Morepreferably the annealing temperature of the hot rolled sheet is950˜1,100° C. The annealing of the hot rolled sheet may be carried outto increase the grain orientation favorable for magnetic properties, asnecessary, but may be omitted.

Subsequently, the hot rolled sheet which was annealed or not is pickled,and cold rolled to a reduction of 70˜95% to obtain a predetermined sheetthickness.

The amounts of added Si, Mn and Al alloy elements that affect coldrollability are appropriately controlled thus attaining superior coldrollability and high reduction. Thus, one cold rolling makes it possibleto form a thin sheet having a thickness of about 0.15 mm. Upon coldrolling, two cold rolling operations including intermediate annealingmay be conducted, as necessary, or two annealing operations may beapplied.

Subsequently, the cold rolled sheet is subjected to final annealing. Ifthe final annealing temperature is lower than 750° C., recrystallizationdoes not sufficiently occur. In contrast, if the final annealingtemperature exceeds 1,100° C., the surface oxide layer is deeply formed,undesirably deteriorating magnetic properties. Hence, final annealing ispreferably conducted at 750˜1,100° C.

The finally annealed steel sheet is subjected to insulation coatingtreatment using typical methods and is then discharged to customers.Upon insulation coating, the application of a typical coating materialis possible, and either Cr-type or Cr-free type may be used withoutlimitation.

Below, the present invention is described by the following examples.Unless otherwise stated, the amounts of components are represented by wt% in the following examples.

Example 1

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 1 below. As such, the amountof each of impurity elements C, S, N, Ti was controlled to 0.002%, and0.3˜0.5% of Al was added to molten steel to facilitate the formation ofinclusions, after which the remainder of Al, and Si and Mn were addedthus making steel ingots. Each of the ingots was heated to 1,150° C.,and finish hot rolled at 850° C. thus manufacturing a hot rolled sheethaving a thickness of 2.0 mm. The hot rolled sheet was annealed at1,050° C. for 4 min and then pickled. Subsequently, cold rolling wasconducted so that the thickness of the sheet was 0.35 mm, followed bycarrying out final annealing at 1,050° C. for 38 sec.

The size and distribution density of inclusions of respective sheets,the core loss, the magnetic flux density and hardness were measured. Theresults are shown in Table 2 below. A sample for use in observing theinclusions was manufactured using a replica method that is typical inthe steel industry, and a transmission electron microscope was usedtherefor. As such, the acceleration voltage of 200 kV was applied.

TABLE 1 Steel Al Si Mn C S N Ti A1 3.0 0.5 1.0 0.002 0.002 0.002 0.002A3 1.0 0.5 1.0 0.002 0.002 0.002 0.002 A4 3.0 1.0 1.0 0.002 0.002 0.0020.002 A5 2.0 1.0 1.0 0.002 0.002 0.002 0.002 A6 1.0 1.0 1.0 0.002 0.0020.002 0.002 A7 0.5 1.0 1.0 0.002 0.002 0.002 0.002 A8 3.5 1.5 1.0 0.0020.002 0.002 0.002 A9 2.5 1.5 1.0 0.002 0.002 0.002 0.002 A10 1.5 1.5 1.00.002 0.002 0.002 0.002 A11 3.0 2.0 1.0 0.002 0.002 0.002 0.002 A13 3.02.5 1.0 0.002 0.002 0.002 0.002 A14 2.5 2.5 1.0 0.002 0.002 0.002 0.002A15 1.0 2.5 1.0 0.002 0.002 0.002 0.002

TABLE 2 Distri. Core Magnetic (Al + Size of Density of Loss Flux Al/ Al/Al + Mn)/ Al + Si + Inclusions Inclusions (W15/ Density Steel Si Mn MnN + S (N + S) Mn/2 (nm) (1/mm²) 50) (B50) Hard. Note A1 6.0 3.0 4.00.0040 1000 4.0 250 0 2.2 1.62 165 Comp. A3 2.0 1.0 2.0 0.0040 500 2.0300 0.02 2.5 1.72 140 Invent. A4 3.0 3.0 4.0 0.0040 1000 4.5 250 0 2.41.62 157 Comp. A5 2.0 2.0 3.0 0.0040 750 3.5 500 0.07 2.0 1.67 155Invent. A6 1.0 1.0 2.0 0.0040 500 2.5 450 0.05 2.1 1.68 150 Invent. A70.5 0.5 1.5 0.0040 375 2.0 50 0 2.5 1.66 145 Comp. A8 2.3 3.5 4.5 0.00401125 5.5 75 0 2.5 1.64 190 Comp. A9 1.7 2.5 3.5 0.0040 875 4.5 400 0.052.0 1.67 185 Invent. A10 1.0 1.5 2.5 0.0040 625 3.5 600 0.08 2.0 1.68170 Invent. A11 1.5 3.0 4.0 0.0040 1000 5.5 250 0 2.3 1.62 195 Comp. A131.2 3.0 4.0 0.0040 1000 6.0 75 0 2.0 1.61 210 Comp. A14 1.0 2.5 3.50.0040 875 5.5 400 0.03 1.9 1.65 190 Invent. A15 0.4 1.0 2.0 0.0040 5004.0 60 0 2.4 1.67 195 Comp.

As is apparent from Table 2, steels A3, A5, A6, A9, A10, and A14 wereinventive examples that satisfy Condition, wherein coarse compositeinclusions having a size of 300 nm or more were observed, and thedistribution density thereof was greater than 0.02(1/mm²) thusexhibiting superior magnetic properties. The Vickers hardness (Hv1) wasas low as 190 or less thus obtaining superior productivity and customerpunchability.

Whereas in steel A1, the ratio of Al+Mn did not satisfy Condition, andthus inclusions having a size of 300 nm or more were not observed, andcore loss and magnetic flux density were deteriorated. Also, in steelA15, the ratio of Al/Si did not satisfy Condition, and thus inclusionshaving a size of 300 nm or more were not observed, and core loss andmagnetic flux density were deteriorated. Also in steels A4, A8, A11 andA13, Al+Mn did not satisfy Condition, and thus inclusions having a sizeof 300 nm or more were not observed, and core loss and magnetic fluxdensity were deteriorated. Also in steel A7, the ratio of Al/Si and theratio of Al/Mn did not satisfy Condition), and thus inclusions having asize of 300 nm or more were not observed, and core loss and magneticflux density were deteriorated.

Example 2

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 3 below. As such, thecomponents of steel were controlled while variously adjusting theamounts of impurity elements N and S, and 0.3˜0.5% of Al was added tomolten steel to facilitate the formation of inclusions, after which theremainder of Al, and Si and Mn were added thus making steel ingots. Eachof the ingots was heated to 1,150° C., and finish hot rolled at 850° C.thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. Thehot rolled sheet was annealed at 1,050° C. for 4 min and then pickled.Subsequently, cold rolling was conducted so that the thickness of thesheet was 0.35 mm, followed by carrying out final annealing at 1,050° C.for 38 sec.

The size and distribution density of inclusions of respective sheets,the core loss, the magnetic flux density and hardness were measured. Theresults are shown in Table 4 below. A sample for observing theinclusions was manufactured using a replica method that is typical inthe steel industry, and a transmission electron microscope was usedtherefor. As such, the acceleration voltage of 200 kV was applied.

TABLE 3 Steel Al Si Mn C S N Ti B1 1.0 0.5 0.5 0.002 0.001 0.001 0.002B3 1.0 0.5 0.5 0.002 0.0005 0.001 0.002 B4 1.0 0.5 1.0 0.002 0.002 0.0030.002 B5 1.2 0.5 1.2 0.002 0.0015 0.002 0.002 B6 1.2 0.5 1.0 0.0020.0005 0.0005 0.002 B7 1.2 0.5 1.0 0.002 0.003 0.003 0.002 B8 2.0 0.52.0 0.002 0.001 0.003 0.002 B9 2.0 0.5 1.5 0.002 0.001 0.0015 0.002 B102.0 0.5 1.5 0.002 0.001 0.003 0.002 B11 2.0 0.5 1.0 0.002 0.003 0.0040.002 B12 2.0 1.0 1.5 0.002 0.0005 0.0015 0.002 B13 2.0 1.0 1.5 0.0020.002 0.004 0.002 B14 1.5 1.0 1.5 0.002 0.002 0.0025 0.002 B15 2.5 1.01.0 0.002 0.0005 0.0005 0.002

TABLE 4 Distri. Core Magnetic (Al + Size of Density of Loss Flux Al/ Al/Al + Mn)/ Al + Si + Inclusions Inclusions (W15/ Density Steel Si Mn MnN + S (N + S) Mn/2 (nm) (1/mm²) 50) (B50) Hard. Note B1 2.0 2.0 1.50.0020 750 1.8 350 0.03 2.6 1.74 135 Invent. B3 2.0 2.0 1.5 0.0015 10001.8 120 0 2.9 1.71 135 Comp. B4 2.0 1.0 2 0.0050 400 2.0 400 0.04 2.61.70 140 Invent. B5 2.4 1.0 2.4 0.0035 686 2.3 450 0.03 2.2 1.69 150Invent. B6 2.4 1.2 2.2 0.0010 2200 2.2 50 0 2.4 1.67 150 Comp. B7 2.41.2 2.2 0.0060 367 2.2 350 0.02 2.3 1.70 165 Invent. B8 4.0 1.0 4.00.0040 1000 3.5 250 0 2.3 1.62 185 Comp. B9 4.0 1.3 3.5 0.0025 1400 3.3450 0.05 2 1.67 170 Invent. B10 4.0 1.3 3.5 0.0040 875 3.3 550 0.08 21.68 170 Invent. B11 4.0 2.0 3 0.0070 429 3.0 250 0 2.2 1.65 170 Comp.B12 2.0 1.3 3.5 0.0020 1750 3.8 80 0 2.3 1.65 165 Comp. B13 2.0 1.3 3.50.0060 583 3.8 500 0.07 2 1.68 175 Invent. B14 1.5 1.0 3 0.0045 667 3.3600 0.07 2 1.68 170 Invent. B15 2.5 2.5 3.5 0.0010 3500 4.0 50 0 2.21.65 165 Comp.

As is apparent from Table 4, steels B1, B4, B5, B7, B9, B10, B13 and B14were inventive examples that satisfy Condition, wherein coarse compositeinclusions having a size of 300 nm or more were observed, and thedistribution density thereof was greater than 0.02(1/mm²) thusmanifesting excellent magnetic properties. The hardness was low thusobtaining superior productivity and customer punchability.

However in steels B3, B6, B11 and B15, N+S fell outside Condition, andthus inclusions having a size of 300 nm or more were not observed, andcore loss and magnetic flux density were deteriorated. Also in steel B8,Al+Mn fell outside Condition, and in steel B12, the ratio of(Al+Mn)/(N+S) fell outside Condition, and thus inclusions having a sizeof 300 nm or more were not observed, and core loss and magnetic fluxdensity were deteriorated.

Example 3

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 5 below. As such, 0.3˜0.5%of Al was added to molten steel to facilitate the formation ofinclusions, after which the remainder of Al, and Si, Mn and P were addedthus making steel ingots. Each of the ingots was heated to 1,150° C.,and finish hot rolled at 850° C. thus manufacturing a hot rolled sheethaving a thickness of 2.0 mm. The hot rolled sheet was annealed at1,050° C. for 4 min and then pickled. Subsequently, cold rolling wasconducted so as to form sheets having different thicknesses in the rangeof 0.15˜0.35 mm, followed by carrying out final annealing at 1,050° C.for 38 sec. The core loss and magnetic flux density of respective sheetshaving different thicknesses were measured. The results are shown inTable 6 below. A sample for observing the inclusions was manufacturedusing a replica method that is typical in the steel industry, and atransmission electron microscope was used therefor. As such, theacceleration voltage of 200 kV was applied.

TABLE 5 Steel Al Si Mn P C S N Ti C1 1 3 0.2 0.03 0.002 0.002 0.0020.002 C2 2.2 1 0.8 0.05 0.002 0.002 0.002 0.002 C3 2 1.5 1.5 0.05 0.0020.002 0.002 0.002 C4 1.8 1.3 1.2 0.05 0.002 0.002 0.002 0.002 C6 2.2 1.50.6 0.1 0.002 0.002 0.002 0.002 C7 1.8 1.2 1.2 0.1 0.002 0.002 0.0020.002

TABLE 6 (Al + Al/ Al/ Al + Mn)/ Al + Si + Magnetic Thickness (mm) SteelSi Mn Mn N + S (N + S) Mn/2 Properties 0.35 0.3 0.25 0.2 0.15 Note C10.3 5.0 1.2 0.004 300 4.1 B50 1.65 1.64 1.63 1.62 1.61 Comp. W10/40020.2 17.8 15.7 13.4 12.3 C2 2.2 2.8 3.0 0.004 750 3.6 B50 1.67 1.66 1.651.64 1.63 Invent. W10/400 18.2 15.6 13.4 11.2 9.7 C3 1.3 1.3 3.5 0.004875 4.25 B50 1.68 1.68 1.65 1.64 1.64 Invent. W10/400 18.0 15 13.6 11.510.1 C4 1.4 1.5 3.0 0.004 750 3.7 B50 1.68 1.65 1.66 1.65 1.63 Invent.W10/400 17.8 15.3 13.3 11.1 9.4 C6 1.5 3.7 2.8 0.004 700 4 B50 1.67 1.661.65 1.64 1.64 Invent. W10/400 18.2 15.6 13.5 11.4 9.8 C7 1.5 1.5 3.00.004 750 3.6 B50 1.68 1.68 1.67 1.66 1.65 Invent. W10/400 19.3 16.514.1 11.7 10

As is apparent from Table 6, steels C2˜C4, C6, C7 were inventiveexamples that satisfy Condition, wherein the magnetic flux density washigh and the core loss was low. This is considered to be because thecomposition according to the present invention had coarsely growninclusions and the distribution density of coarse composite inclusionswas greater than 0.02(1/mm²), and also the texture was stable. Theradio-frequency core loss (W10/400) is surely correlated with thethickness of steel sheet. Specifically, as the thickness of the steelsheet decreases, the properties thereof may be improved. Compared to thesteel sheet having a thickness of 0.35 mm, the core loss of the steelsheet having a thickness of 0.15 mm was improved by about 50%. In steelC1, Al/Mn did not satisfy Condition, and thus core loss (W10/400) andmagnetic flux density (B50) were deteriorated.

Example 4

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 7 below. As such, 0.3˜0.5%of Al was added to molten steel to facilitate the formation ofinclusions, after which the remainder of Al, and Si, Mn and P were addedthus making steel ingots. Each of the ingots was heated to 1,150° C.,and finish hot rolled at 850° C. thus manufacturing a hot rolled sheethaving a thickness of 2.0 mm. The hot rolled sheet was annealed at1,050° C. for 4 min and then pickled. Subsequently, cold rolling wasconducted so that the thickness of the sheet was 0.35 mm, followed bycarrying out final annealing at 1,050° C. for 38 sec.

The size and distribution density of inclusions of respective sheets,the core loss, the magnetic flux density, the Erichsen number andhardness were measured. The results are shown in Table 8 below. A samplefor observing the inclusions was manufactured using a replica methodthat is typical in the steel industry, and a transmission electronmicroscope was used therefor. As such, the acceleration voltage of 200kV was applied.

While the welding part of the hot rolled sheet was pushed-up using asteel ball having a diameter of 20 mm at room temperature, the heightuntil the sheet broken was determined, which is referred to as theErichsen number. The case where the Erichsen number is typically 5 mm ormore makes it possible to produce continuous lines via welding of thesame species.

TABLE 7 Steel Al Si Mn P Sn Sb C S N Ti D1 1.0 2.5 0.5 0.01 — — 0.0020.002 0.002 0.002 D2 2.5 0.8 0.8 0.11 0.03 — 0.002 0.002 0.002 0.002 D32.0 1.3 0.8 0.08 — 0.005 0.002 0.002 0.002 0.002 D4 2.0 1.3 0.8 0.08 —0.03 0.002 0.002 0.002 0.002 D5 2.0 1.3 0.8 0.08 — 0.07 0.002 0.0020.002 0.002 D6 2.0 1.3 0.8 0.08 — 0.1 0.002 0.002 0.002 0.002 D8 1.7 1.60.8 0.08 0.005 — 0.002 0.002 0.002 0.002 D9 1.7 1.6 0.8 0.08 0.03 —0.002 0.002 0.002 0.002 D10 1.7 1.6 0.8 0.08 0.07 — 0.002 0.002 0.0020.002 D11 1.7 1.6 0.8 0.08 0.15 — 0.002 0.002 0.002 0.002 D12 1.7 1.60.8 0.08 0.18 — 0.002 0.002 0.002 0.002 D15 2.2 1.6 0.6 0.05 — 0.030.002 0.002 0.002 0.002 D17 1.5 1.0 1.2 0.19 0.05 — 0.002 0.002 0.0020.002

TABLE 8 Distri. Core Magnetic (Al + Size of Density of Loss Flux Al/ Al/Al + Mn)/ Al + Si + Inclusions Inclusions (W15/ Density Erichsen SteelSi Mn Mn N + S (N + S) Mn/2 (nm) (1/mm²) 50) (B50) (mm) Hard. Note D10.4 2 1.5 0.004 375 3.75 50 0 2.2 1.66 3 204 Comp. D2 3.1 3.1 3.3 0.004825 3.7 600 0.06 2.1 1.67 7 163 Invent. D3 1.5 2.5 2.8 0.004 700 3.7 5000.04 1.9 1.68 7 171 Invent. D4 1.5 2.5 2.8 0.004 700 3.7 540 0.04 1.91.68 9 168 Invent. D5 1.5 2.5 2.8 0.004 700 3.7 600 0.07 1.9 1.68 11 175Invent. D6 1.5 2.5 2.8 0.004 700 3.7 650 0.09 1.9 1.68 8 172 Invent. D81.1 2.1 2.5 0.004 625 3.7 650 0.06 2.1 1.68 8 174 Invent. D9 1.1 2.1 2.50.004 625 3.7 500 0.05 2.0 1.68 10 175 Invent. D10 1.1 2.1 2.5 0.004 6253.7 600 0.08 1.9 1.68 11 177 Invent. D11 1.1 2.1 2.5 0.004 625 3.7 7000.05 2.0 1.68 9 174 Invent. D12 1.1 2.1 2.5 0.004 625 3.7 650 0.04 2.01.68 7 179 Invent. D15 1.4 3.7 2.8 0.004 700 4.1 800 0.12 2.1 1.66 9 178Invent. D17 1.5 1.3 2.7 0.004 675 3.1 550 0.07 2.1 1.69 12 165 Invent.

As is apparent from Table 8, steels D2˜6, D8˜12, D15 and D17 wereinventive examples which satisfy Condition and in which 0.005˜0.2% of Snor 0.005˜0.1% of Sb is added, and thus, the distribution density ofcoarse inclusions having a size of 300 nm or more was greater than0.02(1/mm²), and upon final annealing, the oxide layer and the nitridelayer of the surface were reduced thus improving core loss and magneticflux density. Also, the Erichsen number was high and the Vickershardness (Hv1) was low, thus exhibiting superior weldability,productivity and customer punchability.

Whereas in steel D1, the ratio of Al/Si fell outside Condition, and thusinclusions having a size of 300 nm or more were not observed, and coreloss and magnetic flux density were deteriorated. Also because Sn and Sbwere not added, the Erichsen number was low and weldability wasdecreased and hardness was high undesirably deterioratingprocessability.

Example 5

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 9 below. As such, 0.3˜0.5%of Al was added to molten steel to facilitate the formation ofinclusions, after which the remainder of Al, and Si and Mn were addedthus making steel ingots. Each of the ingots was heated to 1,150° C.,and finish hot rolled at 850° C. thus manufacturing a hot rolled sheethaving a thickness of 2.3 mm. The hot rolled sheet was annealed at1,050° C. for 4 min and then pickled. Subsequently, cold rolling wasconducted so that the thickness of the sheet was 0.50 mm, followed bycarrying out final annealing at 900° C. for 30 sec.

The size and distribution density of inclusions of respective sheets,the core loss, the magnetic flux density and hardness were measured. Theresults are shown in Table 10 below. A sample for observing theinclusions was manufactured using a replica method that is typical inthe steel industry, and a transmission electron microscope was usedtherefor. As such, the acceleration voltage of 200 kV was applied.

TABLE 9 Steel Al Si Mn C S N Ti E2 1.5 0.2 0.5 0.002 0.002 0.002 0.002E3 0.7 0.2 0.5 0.002 0.002 0.002 0.002 E4 2.7 0.5 0.3 0.002 0.002 0.0020.002 E5 1.7 0.5 0.3 0.002 0.002 0.002 0.002 E6 0.7 0.5 0.3 0.002 0.0020.002 0.002 E7 0.5 0.5 0.5 0.002 0.002 0.002 0.002 E8 0.5 0.5 0.5 0.0020.002 0.002 0.002 E9 2.2 0.5 0.2 0.002 0.002 0.002 0.002 E11 1.0 0.1 0.20.002 0.002 0.002 0.002 E14 2.2 0.7 0.2 0.002 0.002 0.002 0.002 E15 0.70.7 0.2 0.002 0.002 0.002 0.002 E16 1.3 0.2 0.7 0.002 0.002 0.002 0.002E19 0.9 0.5 1.0 0.002 0.002 0.002 0.002 E20 0.9 0.7 0.8 0.002 0.0020.002 0.002 E21 1.0 0.5 0.8 0.002 0.002 0.002 0.002

TABLE 10 Distri. Core Magnetic (Al + Size of Density of Loss Flux Al/Al/ Al + Mn)/ Al + Si + Inclusions Inclusions (W15/ Density Steel Si MnMn N + S (N + S) Mn/2 (nm) (1/mm²) 50) (B50) Hard. Note E2 7.5 3.0 2.00.0040 500 2.0 500 0.35 3.0 1.73 140 Invent. E3 3.5 1.4 1.2 0.0040 3001.2 300 0.30 4.0 1.74 110 Invent. E4 5.4 9.0 3.0 0.0040 750 3.4 250 0.013.0 1.68 157 Comp. E5 3.4 5.7 2.0 0.0040 500 2.4 250 0.01 2.9 1.69 145Comp. E6 1.4 2.3 1.0 0.0040 250 1.4 450 0.05 3.5 1.74 115 Invent. E7 1.01.0 1.0 0.0040 250 1.3 50 0.01 4.5 1.74 110 Comp. E8 1.0 1.0 1.0 0.0040250 1.3 75 0.01 4.5 1.74 110 Comp. E9 4.4 11.0 2.4 0.0040 600 2.8 4000.01 2.8 1.68 150 Comp. E11 10 5.0 1.2 0.0040 300 1.2 250 0.01 4.5 1.74105 Comp. E14 3.1 11.0 2.4 0.0040 600 3.0 400 0.01 2.8 1.69 160 Comp.E15 1.0 3.5 0.9 0.0040 225 1.5 150 0.01 3.9 1.74 130 Comp. E16 6.5 1.92.0 0.0040 500 1.9 350 0.25 2.9 1.72 130 Invent. E19 1.8 0.9 1.9 0.0040475 1.9 200 0.01 3.2 1.70 135 Comp. E20 1.3 1.1 1.7 0.0040 425 2.0 3500.05 3.5 1.73 140 Invent. E21 2.0 1.3 1.8 0.0040 450 1.9 400 0.05 3.31.73 140 Invent.

As is apparent from Table 10, steels E2, E3, E6, E16, E20 and E21 wereinventive examples that satisfy Condition, wherein the coarse inclusionshaving a size of 300 nm or more were observed, and the distributiondensity thereof was greater than 0.02(1/mm²) thus exhibiting superiormagnetic properties, and the Vickers hardness (Hv1) was 140 or less,resulting in good productivity and customer punchability.

Whereas in steels E4, E5, E9, E11, E14, E15 and E19, the ratio of Al/Mnand the amount of Al+Mn fell outside Condition and thus inclusionshaving a size of 300 nm or more were not observed, and core loss andmagnetic flux density were deteriorated.

Example 6

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 11 below. As such, 0.3˜0.5%of Al was added to molten steel to facilitate the formation ofinclusions, after which the remainder of Al, and Si and Mn were addedthus making steel ingots. Each of the ingots was heated to 1,150° C.,and finish hot rolled at 850° C. thus manufacturing a hot rolled sheethaving a thickness of 2.3 mm. The hot rolled sheet was annealed at1,050° C. for 4 min and then pickled. Subsequently, cold rolling wasconducted so that the thickness of the sheet was 0.50 mm, followed bycarrying out final annealing at 900° C. for 30 sec.

The size and distribution density of inclusions of respective sheets,the core loss, the magnetic flux density and hardness were measured. Theresults are shown in Table 12 below. A sample for observing theinclusions was manufactured using a replica method that is typical inthe steel industry, and a transmission electron microscope was usedtherefor. As such, the acceleration voltage of 200 kV was applied.

TABLE 11 Steel Al Si Mn C S N Ti F1 1.0 0.5 0.3 0.0030 0.0010 0.00100.0020 F2 0.7 0.3 0.2 0.0030 0.0030 0.0030 0.0020 F4 0.7 0.5 0.3 0.00300.0010 0.0025 0.0020 F5 1.0 0.3 0.7 0.0030 0.0005 0.0005 0.0020 F6 1.00.3 0.7 0.0030 0.0040 0.0020 0.0020 F8 1.2 0.2 0.3 0.0030 0.0015 0.00100.0020 F9 0.9 0.5 0.8 0.0030 0.0020 0.0020 0.0020 F10 0.9 0.5 0.8 0.00300.0040 0.0030 0.0020 F11 0.9 0.5 0.5 0.0030 0.0030 0.0030 0.0020 F12 0.90.5 0.5 0.0030 0.0020 0.0025 0.0020 F13 0.9 0.5 0.5 0.0030 0.0005 0.00050.0020

TABLE 12 Distri. Core Magnetic (Al + Size of Density of Loss Flux Al/Al/ Al + Mn)/ Al + Si + Inclusions Inclusions (W15/ Density Steel Si MnMn N + S (N + S) Mn/2 (nm) (1/mm²) 50) (B50) Hard. Note F1 2.0 3.3 1.30.0020 650 1.7 350 0.150 3.2 1.73 135 Invent. F2 2.3 3.5 0.9 0.0060 1501.1 200 0.010 4.2 1.71 130 Comp. F4 1.4 2.3 1 0.0035 286 1.4 450 0.0503.4 1.73 130 Invent. F5 3.3 1.4 1.7 0.0010 1700 1.7 50 0.010 3.5 1.69140 Comp. F6 3.3 1.4 1.7 0.0060 283 1.7 350 0.200 3.2 1.74 140 Invent.F8 6.0 4.0 1.5 0.0025 600 1.6 450 0.070 3.3 1.74 140 Invent. F9 1.8 1.11.7 0.0040 425 1.8 550 0.080 3.1 1.73 135 Invent. F10 1.8 1.1 1.7 0.0070243 1.8 250 0.010 3.5 1.69 135 Comp. F11 1.8 1.8 1.4 0.0060 233 1.7 5000.150 3.2 1.73 135 Invent. F12 1.8 1.8 1.4 0.0045 311 1.7 600 0.180 3.21.74 135 Invent. F13 1.8 1.8 1.4 0.0010 1400 1.7 50 0.018 3.7 1.72 135Comp.

As is apparent from Table 12, steels F1, F4, F6, F8, F9, F11 and F12were inventive examples that satisfy Condition, wherein the coarseinclusions having a size of 300 nm or more were observed, and thedistribution density thereof was greater than 0.02(1/mm²) thusexhibiting superior magnetic properties, and hardness was low, resultingin good productivity and customer punchability.

Whereas in steels F5, F10 and F13, the amount of N+S fell outsideCondition and thus inclusions having a size of 300 nm or more were notobserved, and core loss and magnetic flux density were deteriorated.

Example 7

Vacuum melting was performed in a laboratory, thus preparing steelingots having the components shown in Table 13 below. As such, 0.3˜0.5%of Al was added to molten steel to facilitate the formation ofinclusions, after which the remainder of Al, and Si and Mn were addedthus making steel ingots. Each of the ingots was heated to 1,150° C.,and finish hot rolled at 850° C. thus manufacturing a hot rolled sheethaving a thickness of 2.0 mm. The hot rolled sheet was annealed at1,050° C. for 4 min and then pickled. Subsequently, cold rolling wasconducted so that the thickness of the sheet was 0.35 mm, followed bycarrying out final annealing at 1,050° C. for 38 sec.

The size and distribution density of inclusions of respective sheets,the core loss, the magnetic flux density and hardness were measured. Theresults are shown in Table 14 below. A sample for observing theinclusions was manufactured using a replica method that is typical inthe steel industry, and a transmission electron microscope was usedtherefor. As such, the acceleration voltage of 200 kV was applied.

TABLE 13 Steel Al Si Mn C S N Ti G1 3.0 2.3 1.0 0.002 0.002 0.002 0.002G7 0.5 2.7 0.8 0.002 0.002 0.002 0.002 G8 3.5 3.0 0.8 0.002 0.002 0.0020.002 G11 3.0 3.2 1.0 0.002 0.002 0.002 0.002 G13 3.0 2.5 1.0 0.0020.002 0.002 0.002 G14 2.5 2.5 1.0 0.002 0.002 0.002 0.002

TABLE 14 Distri. Core Magnetic (Al + Size of Density of Loss Flux Al/Al/ Al + Mn)/ Al + Si + Inclusions Inclusions (W15/ Density Steel Si MnMn N + S (N + S) Mn/2 (nm) (1/mm²) 50) (B50) Hard. Note G1 1.3 3.0 4.00.0040 1000 5.8 250 0.01 2.0 1.62 225 Comp. G7 0.2 0.6 1.3 0.0040 3253.6 50 0.01 2.5 1.66 190 Comp. G8 1.2 4.4 4.3 0.0040 1075 6.9 75 0.012.0 1.62 230 Comp. G11 0.9 3.0 4.0 0.0040 1000 6.7 250 0.005 2.3 1.62230 Comp. G13 1.2 3.0 4.0 0.0040 1000 6.0 75 0.01 2.0 1.62 220 Comp. G141.0 2.5 3.5 0.0040 875 5.5 400 0.10 2.1 1.64 225 Invent.

As is apparent from Table 14, steel G14 was inventive examples thatsatisfy Condition wherein the coarse inclusions having a size of 300 nmor more were observed, and the distribution density thereof was greaterthan 0.02(1/mm²) thus exhibiting superior magnetic properties, and theVickers hardness was as low as 225 or less.

Whereas in steels G1, G8, G11 and G13, the amount of Al+Mn fell outsideCondition and thus inclusions having a size of 300 nm or more were notobserved, and core loss and magnetic flux density were deteriorated. Insteel G7, Al/Si, Al/Mn, did not satisfy Condition, and thus inclusionshaving a size of 300 nm or more were not observed, and core loss andmagnetic flux density were deteriorated. In steels G8 and G11, Al+Mn didnot satisfy Condition, and thus hardness was high, thereby deterioratingproductivity and punchability.

1. A non-oriented electrical steel, comprising 0.7˜2.5% of Al, 0.2˜2.5%of Si, 0.2˜1.5% of Mn, 0.0005˜0.004% of N, 0.0005˜0.004% of S, 0.003% orless of C, and a balance of Fe and other inevitable impurities by wt %,and satisfying the conditions below: Conditions: 1≤{[Al]+[Mn]}≤3.50.002≤{[N]+[S]}≤0.006, 250≤{([Al]+[Mn])/([N]+[S])}≤1,400,1.4≤{[Al]+[Si]+[Mn]/2}≤5.5, 1≤[Al]/[Si]≤7.5, and 1≤[Al]/[Mn]≤3.7,wherein [Al], [Si], [Mn], [N] and [S] indicate amounts (wt %) of Al, Si,Mn, N and S, respectively.
 2. The non-oriented electrical steel sheet ofclaim 1, further comprising 0.2% or less of P.
 3. The non-orientedelectrical steel sheet of claim 1, further comprising at least one of0.005˜0.2% of Sn and 0.005˜0.1% of Sb.
 4. The non-oriented electricalsteel sheet of claim 1, further comprising 0.004% or less of Ti.
 5. Thenon-oriented electrical steel sheet of claim 1, wherein an inclusioncomprising a nitride and a sulfide alone or a combination thereof isformed in the steel sheet.
 6. The non-oriented electrical steel sheet ofclaim 5, wherein the nitride comprises aluminum nitride and the sulfidecomprises magnesium sulfide.
 7. The non-oriented electrical steel sheetof claim 5, wherein the inclusion having an average size of 300 nm ormore.
 8. The non-oriented electrical steel sheet of claim 5, wherein adistribution density of the inclusion having a size of 300 nm or more isequal to or greater than 0.02 number/mm².
 9. The non-oriented electricalsteel sheet of claim 1, wherein a cross-sectional Vickers hardness (Hv1)is 190 or less.